In the quest for developing novel and efficient batteries, a great interest has been raised for sustainable K-based honeycomb layer oxide materials, both for their application in energy devices as well as for their fundamental material properties. A key issue in the realization of efficient batteries based on such compounds, is to understand the K-ion diffusion mechanism. However, investigation of potassium-ion (K+) dynamics in materials using e.g. NMR and related techniques has so far been very challenging, due to its inherently weak nuclear magnetic moment, in contrast to other alkali ions such as lithium and sodium. Spin-polarised muons, having a high gyromagnetic ratio, make the muon spin rotation and relaxation (mu+SR) technique ideal for probing ions dynamics in these types of energy materials. Here we present a study of the low-temperature magnetic properties as well as K+ dynamics in honeycomb layered oxide material K2Ni2TeO6 using mainly the mu+SR technique. Our low-temperature mu+SR results together with complementary magnetic susceptibility measurements find an antiferromagnetic transition at T-N approximate to 27 K. Further mu+SR studies performed at higher temperatures reveal that potassium ions (K+) become mobile above 200 K and the activation energy for the diffusion process is obtained as E-a = 121(13) meV. This is the first time that K+ dynamics in potassium-based battery materials has been measured using mu+SR. Assisted by high-resolution neutron diffraction, the temperature dependence of the K-ion self diffusion constant is also extracted. Finally our results also reveal that K-ion diffusion occurs predominantly at the surface of the powder particles. This opens future possibilities for potentially improving ion diffusion as well as K-ion battery device performance using nano-structuring and surface coatings of the particles.

In this work we present systematic set of measurements carried out by muon spin rotation/relaxation (μ+SR) and neutron powder diffraction (NPD) on the solid solution NaxCa1−xCr2O4. This study investigates Na-ion dynamics in the quasi-1D (Q1D) diffusion channels created by the honeycomb-like arrangement of CrO6 octahedra, in the presence of defects introduced by Ca doping. With increasing Ca content, the size of the diffusion channels is enlarged, however, this effect does not enhance the Na ion mobility. Instead the overall diffusivity is hampered by the local defects and the Na hopping probability is lowered. The diffusion mechanism in NaxCa1−xCr2O4 was found to be interstitial and the activation energy as well as diffusion coefficient were determined for all the members of the solid solution. 

Two−dimensional (2D) triangular lattices antiferromagnets (2D−TLA) often manifest intriguing physical and technological properties, due to the strong interplay between lattice geometry and electronic properties. The recently synthesized 2−dimensional transition metal dichalcogenide LiCrTe2, being a 2D−TLA, enriched the range of materials which can present such properties. In this work, muon spin rotation (μ+SR) and neutron powder diffraction (NPD) have been utilized to reveal the true magnetic nature and ground state of LiCrTe2. From high−resolution NPD the magnetic spin order at base−temperature is not, as previously suggested, helical, but rather collinear antiferromagnetic (AFM) with ferromagnetic (FM) spin coupling within the ab−plane and AFM coupling along the c−axis. The ordered magnetic Cr moment is established as μCr= 2.36 μB. From detailed μ+SR measurements we observe an AFM ordering temperature TN≈ 125 K. This value is remarkably higher than the one previously reported by magnetic bulk measurements. From μ+SR we are able to extract the magnetic order parameter, whose critical exponent allows us to categorize LiCrTe2 in the 3D Heisenberg AFM universality class. Finally, by combining our magnetic studies with high−resolution synchrotron X−ray diffraction (XRD), we find a clear coupling between the nuclear and magnetic spin lattices. This suggests the possibility for a strong magnon−phonon coupling, similar to what has been previously observed in the closely related compound LiCrO2.